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I was watching an episode of Engine Masters on the Motortrend channel. David Freiburger was talking about Helmholtz Resonance.

What is Helmholtz Resonance and how does it apply to engine building?

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The basic idea of a Helmholtz resonator (invented by Helmholtz in the 19th century, for experiments into acoustics) is an enclosed chamber, with a large hole in one side and a narrow neck ending in a small hole at the opposite side.

The resonant frequency of the air inside the chamber depends on its total volume, the length of the neck, and the cross sectional area of the small hole (but not on the dimensions of the large hole).

An exhaust system which includes such a device will tend to amplify the sound at the resonant frequency, changing the quality of the exhaust noise.

Similar devices have also been used in air intakes, to change the way that the inlet pressure varies with engine RPM, but there is so much handwaving about the general subject that it is hard to separate out the physics from the snake oil.

For example the resonator effect in a air intake could mean that at a particular RPM, the amount of "pulsing" in the air flow on the inlet side is reduced because the air inside the resonator is vibrating at the right frequency to supply "pulses" of air to the cylinders on each inlet stroke. Whether that makes any practical difference to engine efficiency is a different question, though it may reduce the noise level from the air intake over a particular range of RPMs.

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    In the 80's Yamaha added external chambers to the induction circuit on a number of their 4-stroke motorcycles (YICS) that would pulse a jet of stored air at high vacuum and low revs to swirl the charge in the combustion chamber for smooth idling. It was a tuned circuit but the chamber was connected by arterial tubes. It was a compact implementation based upon the same theory.
    – mckenzm
    Commented Oct 14, 2019 at 1:10
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This could be an issue with either the air intake plenum chamber or a given silencer as the resonance can set up vibrations to damage or split the device.

iirc David Vizzard looked at this when tuning exhaust & manifold for mini engines back in the 70's...

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